Method and apparatus for association control in mobile wireless networks

ABSTRACT

A method for association of a mobile terminal with an access point (AP) includes determining a set of available APs. The AP from among the available APs that has the coverage area that is likely to encompass the mobile terminal for the greatest period of time or distance is selected. The selected AP is associated with the mobile terminal.

BACKGROUND OF THE INVENTION

1. Technical Field

The present disclosure relates to wireless networks and, morespecifically, to a method and apparatus for association control inmobile wireless networks.

2. Discussion of the Related Art

A mobile wireless network is a set of spatially distributed basestations or access points (APs) that are each connected to a network,for example, a telephone network and/or the Internet. Each AP provides acoverage area within which it is able to communicate wirelessly to amobile terminal and thus provide the mobile terminal with networkaccess. As the mobile terminal travels beyond the coverage area of afirst AP and into the coverage area of a second AP, networkcommunication is handed off from the first AP to the second AP such thatthe mobile terminal may retain network access even when in motion.

Examples of mobile wireless networks include mobile telephone networkssuch as CDMA and GSM networks, networks based on IEEE 802.11 standards(WiFi), networks based on IEEE 802.16 standards (WiMAX), etc.

Handoff is performed as the mobile terminal travels beyond the coveragearea of the AP currently providing network access. Specifically, handoffis performed before the signal received by the mobile terminal from theAP degrades below a point where network access is compromised. Thispoint may be determined by monitoring the signal strength of the currentAP and initiating a handoff when the signal strength of the current APfalls below a signal strength of an available AP.

Handoff of network access from one AP to another AP involves, amongother concerns, determining which AP to handoff network access to. Forexample, the mobile terminal may be within the coverage area of multipleAPs and there may be a selection of APs that may receive the handoff.

Conventionally, network access is handed off to the AP with thestrongest signal. In such a situation, the mobile terminal may analyzethe signal strength of each available AP and select a desired AP basedon the strongest signal. Accordingly, as the mobile terminal leaves thecoverage area of a first AP, network access may be handed off to anotherAP without a loss of network access.

However, frequent handoff of network access may be problematic for amobile terminal. For example, handoffs may potentially causeinterruption of data transfer across the network. When data transfer isinvolved in real-time communication service, such as a voice call,interruption of data transfer may be undesirable potentially leading tonoticeable delays in communication. Additionally, interruption of datatransfer may lead to unacceptable delays in interactive applicationsthat rely upon network-based communication. Moreover, the process ofhanding off network access may be expensive in terms of battery drainand computational economy, both of which may be scarce in mobileterminals.

Approaches have been developed to minimize latency in handoffs. Forexample, S. Pack and Y. Choi, “Fast Inter-AP Handoff using PredictiveAuthentication Scheme in a Public Wireless LAN,” in proc. IEEE NetworksConference, Atlanta, Ga., August 2002; M. Shin, A. Mishra, and W.Arbaugh, “Improving the Latency of 802.11 Hand-offs using NeighborGraphs,” in Proc. ACM MobiSys, Boston Mass., June 2004; A. Mishra, M.Shin, and W. Arbaugh, “Context Caching using Neighbor Graphs for FastHandoffs in a Wireless Network,” in Proc. IEEE Infocom, Hong Kong,China, March 2004; and I. Ramani and S. Savage, “SyncScan: PracticalFast Handoff for 802.11 Infrastructure Networks,” in Proc. IEEE Infocom,Miami, Fla., March 2005; each of which are herein incorporated byreference. According to such approaches, channel scanning time andhandoff latency may be minimized. However, such approaches have not beenconcerned with minimizing the number of handoffs, and accordingly,expense of battery drain and computational economy associated withhandoffs is not minimized.

SUMMARY

A method for association of a mobile terminal with an access point (AP),includes determining a set of presently available APs, determining whichof the set of presently available APs were also within a set ofpreviously available APs, selecting an AP from among the intersection ofthe set of presently available APs and the set of previously availableAPs, and associating the mobile terminal with the selected AP.

The set of presently available APs may include APs of a wireless networkthat have a coverage area that encompasses the mobile terminal at thepoint in time when the set is determined.

The previously available APs may be APs of a wireless network that havea coverage area that encompassed the mobile terminal at a particularpoint in time before the point in time when the set is determined.

The selecting of an AP from among the intersection of the set ofpresently available APs and the set of previously available APs mayinclude making a random selection from among the intersection.

The selecting of an AP from among the intersection of the set ofpresently available APs and the set of previously available APs mayinclude selecting an AP of the intersection whose signal is moststrongly received by the mobile terminal.

The association of the mobile terminal with the selected AP may be aninitial association when the mobile terminal has no previous associationand the association of the mobile terminal with the selected AP may be ahandoff when the mobile terminal has a previous association.

The previously available APs may have been APs available during a mostrecently marked time slot that occurred when the intersection of the setof presently available APs and the set of previously available APs was anull set.

A method for association of a mobile terminal with an access point (AP)includes determining a set of available APs at a first location,determining a set of available APs at a second location, retrievinghistory data pertaining to a transition between the set of available APsat the first location and the set of available APs at the secondlocation, the history data including association duration informationindicating how long each of the available APs at the first locationremained available, selecting an AP that has the greatest associationduration from among the set of available APs at the second location, andassociating the mobile terminal with the selected AP.

The set of available APs at the first location may include APs of awireless network that have a coverage area that encompasses the mobileterminal when located at the first location and the set of available APsat the second location includes APs of the wireless network that have acoverage area that encompasses the mobile terminal when located at thesecond location.

The second location may be a present location of the mobile terminal andthe first location is a previous location of the mobile terminal and thehistory data pertains to a prior transition of the mobile terminal fromthe first location to the second location.

Selecting an AP that has the greatest association duration from amongthe set of available APs at the second location may include randomlyselecting between two or more APs that have the same associationduration.

Selecting an AP that has the greatest association duration from amongthe set of available APs at the second location may include selectingbetween two or more APs that have the same association duration based onthe AP whose signal is most strongly received by the mobile terminal.

The association of the mobile terminal with the selected AP may be aninitial association when the mobile terminal has no previous associationand the association of the mobile terminal with the selected AP may be ahandoff when the mobile terminal has a previous association.

A method for association of a mobile terminal with an access point (AP)includes determining a set of available APs, calculating an extent ofuninterrupted reception for each of the available APs, selecting an APfrom among the set of available APs that has a greatest extent ofuninterrupted reception, and associating the mobile terminal with theselected AP.

The set of available APs may include APs of a wireless network that havea coverage area that encompasses the mobile terminal at the point intime when the set is determined.

The extent of uninterrupted reception may be calculated for each of theavailable APs by determining how long the mobile terminal will remainwithin the coverage area of each of the APs given a known or estimatedtrajectory of the mobile terminal.

Selecting an AP from among the set of available APs that has a greatestextent of uninterrupted reception may include randomly selecting an APfrom among APs that have an identical extent of uninterrupted reception.

Selecting an AP from among the set of available APs that has a greatestextent of uninterrupted reception may include selecting an AP having agreatest signal strength from among APs that have an identical extent ofuninterrupted reception.

The association of the mobile terminal with the selected AP may be aninitial association when the mobile terminal has no previous associationand the association of the mobile terminal with the selected AP may be ahandoff when the mobile terminal has a previous association.

A method for association of a mobile terminal with an access point (AP)includes determining a set of available APs, determining which of theset of available APs has a coverage area that is likely to encompass themobile terminal for the greatest period of time or distance, selectingthe AP from among the available APs that has the coverage area that islikely to encompass the mobile terminal for the greatest period of timeor distance, and associating the mobile terminal with the selected AP.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present disclosure and many of theattendant features thereof will be described as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a diagram showing network of multiple access points (APs) thatmay be used to provide network access to a mobile terminal in accordancewith an exemplary embodiment of the present invention;

FIG. 2 is a flowchart illustrating a method for performing AP handoffusing a Lookahead approach according to an exemplary embodiment of thepresent invention;

FIG. 3 is a diagram showing an application of a Lookback approachapplied to a network of multiple access points (APs) that may be used toprovide network access to a mobile terminal in accordance with anexemplary embodiment of the present invention;

FIG. 4 is a flowchart illustrating a method for performing AP handoffusing a Lookback approach according to an exemplary embodiment of thepresent invention;

FIG. 5 is a diagram showing network of multiple access points (APs) thatmay be used to provide network access to a mobile terminal in accordancewith an exemplary embodiment of the present invention;

FIG. 6 is a flow chart illustrating a method for performing AP handoffusing a Track approach according to an exemplary embodiment of thepresent invention; and

FIG. 7 shows an example of a computer system which may implement amethod and system of the present disclosure.

DETAILED DESCRIPTION OF THE DRAWINGS

In describing the exemplary embodiments of the present disclosureillustrated in the drawings, specific terminology is employed for sakeof clarity. However, the present disclosure is not intended to belimited to the specific terminology so selected, and it is to beunderstood that each specific element includes all technical equivalentswhich operate in a similar manner.

Exemplary embodiments of the present invention provide approaches forminimizing the frequency of handoffs as a mobile terminal roams througha mobile communications network. The frequency of handoffs may beminimized by utilizing one or more of the approaches discussed hereinfor selecting a desired AP to receive handoff. By minimizing thefrequency of handoffs, the quality of mobile network communication maybe maximized while battery power and computational resources of themobile terminal may be conserved.

As discussed above, the conventional approach for selecting an AP toreceive handoff includes determining which of the available APs providesthe strongest signal to the mobile terminal and then handing off networkaccess to the AP that has been determined to provide the strongestsignal. The strength of the signal may primarily depend on the distancebetween the mobile terminal and the access point, but other factors suchas obstructions and atmospheric conditions may play a role as well. Forthe purposes of simplifying discussion, it will be assumed herein thatsignal strength is wholly a factor of distance between the mobileterminal and the access point.

Thus according to the conventional approach, when a mobile terminalroaming within a network of APs leaves the coverage area of a particularAP, network access is handed off to the AP that is closest to the mobileterminal at that moment. FIG. 1 is a diagram showing network handoff.The trajectory of the mobile terminal is assumed to be linear for thebenefit of description.

In FIG. 1, the mobile terminal MT follows a trajectory T. At location L1along the trajectory T, the mobile terminal MT determines that while itis within the coverage area of a first access point AP1 and a secondaccess point AP2, a network connection is established with AP1 becauseat L1, the signal from AP1 is stronger than the signal from AP2. Thusnetwork access is initially provided by AP1.

As the mobile terminal MT proceeds to L2, and the mobile terminal MT iswithin the coverage areas of AP1, AP2 and AP3, the mobile terminal MTdetermines that the signal is strongest from AP2. Thus network access ishanded off from AP1 to AP2 (handoff 1).

As the mobile terminal MT proceeds to L3, and the mobile terminal MT iswithin the coverage areas of AP2, AP3 and AP4, the mobile terminal MTdetermines that the signal is strongest from AP3. Thus network access ishanded off from AP2 to AP3 (handoff 2).

As the mobile terminal MT proceeds to L4, and the mobile terminal MT iswithin the coverage areas of AP3, AP4 and AP5, the mobile terminal MTdetermines that the signal is strongest from AP4. Thus network access ishanded off from AP3 to AP4 (handoff 3).

As the mobile terminal MT proceeds to L5, and the mobile terminal MT iswithin the coverage areas of AP4, AP5 and AP7, the mobile terminal MTdetermines that the signal is strongest from AP5. Thus network access ishanded off from AP4 to AP5 (handoff 4).

As the mobile terminal MT proceeds to L6, and the mobile terminal MT iswithin the coverage areas of AP5, AP6 and AP7, the mobile terminal MTdetermines that the signal is strongest from AP6. Thus network access ishanded off from AP5 to AP6 (handoff 5).

As the mobile terminal MT proceeds to L7, and the mobile terminal MT iswithin the coverage areas of AP6, AP7 and AP8, the mobile terminal MTdetermines that the signal is strongest from AP7. Thus network access ishanded off from AP6 to AP7 (handoff 6).

As the mobile terminal MT proceeds to L8, and the mobile terminal MT iswithin the coverage areas of AP7, AP8 and AP9, the mobile terminal MTdetermines that the signal is strongest from AP8. Thus network access ishanded off from AP7 to AP8 (handoff 7).

As the mobile terminal MT proceeds to L9, and the mobile terminal MT iswithin the coverage areas of AP8, AP9 and AP10, the mobile terminal MTdetermines that the signal is strongest from AP9. Thus network access ishanded off from AP8 to AP9 (handoff 8).

Accordingly, as the mobile terminal MT progresses along the trajectory Tfrom L1 to L9, there are a total of 8 handoffs performed when using theconventional approach for handing off.

To minimize the frequency associated with access point handoff,exemplary embodiments employ techniques for selecting access points thatare most likely to maximize duration of association. Among the varioustechniques for selecting among multiple APs contemplated by exemplaryembodiments of the present invention, three exemplary techniques arediscussed in detail below. These techniques are Lookahead, Lookback, andTrack. The Lookahead approach involves an offline approach wherein thetrajectory of the mobile terminal is known or may be assumed. TheLookback and Track approaches involve online approaches wherein thetrajectory of the mobile terminal is unknown.

Lookahead Approach

As discussed above, the Lookahead approach to AP selection is an offlineapproach where the trajectory of the mobile terminal is known or may beassumed. For example, the Lookahead approach may be used where themobile terminal is moving along a knowable path, such as in a trainalong a track or an automobile along a highway. The mobile terminal'sknown trajectory need not be a straight line, all that is needed is thatthe trajectory be known or assumed. The trajectory of the mobile devicemay either be preprogrammed by a user or determined/estimated by themobile device, for example, by analyzing a pattern of past APassociation. Global positioning service technology and/or a mapping ofthe location of available APs may also be used in figuring out thetrajectory of the mobile terminal, although this information is notrequired in all exemplary embodiments.

In performing the lookahead approach, handoff is triggered by thedropping off of signal strength below a predetermined threshold belowwhich it is known that signal strength is undesirable. When the handoffis triggered, the mobile terminal may determine a list of available APswhose signal strength is above the predetermined threshold. For eachavailable AP, the known and/or estimated trajectory is used to determinea duration value representing the extent of uninterrupted receptionabove the predetermined threshold either in terms of time (wherevelocity of the mobile terminal is known) or in terms of distance. Then,the available AP with the greatest measure of uninterrupted reception isselected and network access is handed off to the selected AP.

In determining the extent of uninterrupted reception above thepredetermined threshold, proximity may be used as a proxy for receptionstrength, and reception strength above a predetermined threshold may beexpressed as a maximum distance from a particular access point. Whereadditional data is available, such as obstructions and atmosphericconditions that may affect a signal's ability to carry, this data may beused to augment the calculations.

Determining the extent of uninterrupted reception may be performed fordiscrete time slots of predetermined time intervals, for example,several seconds. Using the trajectory of the mobile terminal, the levelof estimated reception for each available access point may be calculatedfor a predetermined number of time slots, for example 10 time slots.

For example, it may be determined for each available access point, whatthe distance will be between that access point and the mobile terminalat each time slot. If a particular AP is determined to be within thepredetermined distance of the mobile terminal for all 10 future timeslots, then the extent of uninterrupted reception for that AP is 10. Ifa particular AP is determined to be within the predetermined distance ofthe mobile terminal for the first 5 time slots and then beyond thepredetermined distance for the next 5 time slots then the extent ofuninterrupted reception for that AP is 6. However, if a particular AP isdetermined to be within the predetermined distance of the mobileterminal for the first 3 time slots, beyond the predetermined distancefor the next 2 time slots and then back within the predetermineddistance of the mobile terminal for the remaining 5 time slots, then theextent of uninterrupted reception for that AP is 3, because it is onlywithin the first 3 time slots that uninterrupted reception from theinitial time slot is achieved.

Accordingly, the AP with the longest measure of uninterrupted receptionis selected and network access is handed off to the selected AP.

Thus returning to FIG. 1, the Lookahead approach for AP selection may beused to minimize the number of handoffs. For example, at location L1along the trajectory T, the mobile terminal MT determined that it iswithin the coverage area of AP1 and AP2. Based on the trajectory T, forAP1, reception will be present at L1, present at L2, and not present atL3. Thus, the extent of uninterrupted reception for AP1 is 2. Meanwhile,for AP2, reception will be present at L1, L2 and L3. Thus the extent ofuninterrupted reception for AP2 is 3. Because the extent ofuninterrupted reception for AP2 exceeds the comparable figure for AP1,AP2 is selected and thus network access is initially provided by AP2.

As the mobile terminal MT proceeds to L2 and L3, no handoff isperformed.

As the mobile terminal MT proceeds to L4, and reception from AP2 fallsbellow the threshold level, the MT determines that it is within thecoverage area of AP3, AP4 and AP5. Based on an analysis similar to theanalysis described above, AP5 is selected for having the greatest extentof uninterrupted reception and handoff is performed to AP5 (handoff 1).

As the mobile terminal MT proceeds to L5 and L6, no handoff isperformed.

As the mobile terminal MT proceeds to L7, and the reception from AP5falls bellow the threshold level, the MT determines that it is withinthe coverage area of L6, L7 and L8. Based on an analysis similar to theanalysis described above, AP8 is selected for having the greatest extentof uninterrupted reception and handoff is performed to AP8 (handoff 2).

As the mobile terminal MT proceeds to L8 and L9, no handoff isperformed.

Accordingly, as the mobile terminal MT progresses along the trajectory Tfrom L1 to L9, there are a total of 2 handoffs performed when using aLookahead approach according to an exemplary embodiment of the presentinvention. This stands in contrast to the 8 handoffs used according tothe conventional approach.

FIG. 2 is a flowchart illustrating a method for performing AP handoffusing a Lookahead approach according to an exemplary embodiment of thepresent invention. First, the mobile terminal may determine what APs areavailable (Step S21). Then, for each available AP, the mobile terminalmay calculate the extent of uninterrupted reception based on a known orassumed trajectory (Step S22). The extent of uninterrupted reception maybe calculated as the length of time and/or travel distance that themobile terminal will remain within the coverage area of the particularAP without interruption, starting from the time and location where themobile terminal determined what APs were available. The available APwith the greatest extent of uninterrupted reception may then be selected(Step S23). Where there are multiple APs with an identical greatestextent of uninterrupted reception, selection may be arbitrary from amongthe group of APs that share the greatest extent of uninterruptedreception. Alternatively, selection from among the group of APs thatshare the greatest extent of uninterrupted reception may be based uponwhich of the group of APs has the greatest signal strength. Where the APis selected for the purposes of establishing initial network access,initial network access may be established with the selected AP. Wherethe AP is selected for the purposes of handing off network access,network access may be handed off to the selected AP (Step S24). Afternetwork access is established and/or handed off, the signal strength maybe monitored to determine if the signal strength is within apredetermined threshold (Step S25). For as long as the signal strengthis within the predetermined threshold (Yes, S25), no handoff isnecessary and monitoring may continue (Step S25). When the signalstrength falls below the predetermined threshold (No, S25), then thehandoff process may be repeated at Step S21.

Lookback Approach

As discussed above, the Lookback approach to AP selection is an onlineapproach where the trajectory of the mobile terminal is unknown.

Even though the trajectory of the mobile terminal is unknown, exemplaryembodiments of the present invention may provide techniques forminimizing the number of handoffs. The lookback approach works on theassumption that as the mobile terminal roams, an access point that has arecent history of covering the mobile terminal will be more likely tocontinue to cover the mobile terminal than an access point that has justbegun to cover the mobile terminal.

This concept may be described by example. The chart of FIG. 3 shows howthe lookback approach may be performed with respect to a particularexample. In this example, it is assumed that the mobile terminal istraveling along an unknown trajectory. There are 12 potential APsnumbered AP1 through AP12. There are also 9 time slots. A time slot maybe defined in a number of ways. For example, a time slot may be a fixedtime interval. Alternatively, time slots may depend on the occurrence ofa particular event, such as when a scan for available APs is performed.According to this example, every time the mobile terminal performsanother scan for available APs, a new time slot has begun. Under someimplementations of this approach, a new scan is performed whenever ahandoff is required, however, there may be other events that trigger ascan as well.

In FIG. 3, an “x” indicates which APs are available at which time slots,for example, AP1 is available to the mobile terminal at time slot 1,while AP12 is not available at time slot 1. Square brackets “[ ]” areused to indicate which of the available APs is the associated AP at anygiven time slot.

Some time slots are considered to be “marked” time slots, these timeslots receive an asterisk “*” in the “MARK” row. The concept of markingis described in detail below.

At time slot 1, there is no previous association so an initialassociation is determined. The initial association may be determinedaccording to either a random selection from among the available APs orby selecting the AP with the strongest signal. In the example shown inFIG. 3, AP2 is so selected and an initial association is begun.

The time slot in which an initial association is begun should be markedand thus time slot 1 is a marked time slot. As the mobile terminalroams, its position changes, and in time slot 2, the associated AP, AP2is no longer available. Because the associated AP is no longeravailable, handoff is performed.

The selection of the appropriate AP to handoff to is the subject of thelookback approach. According to the lookback approach, network access ishanded off to an AP that is in the intersection of the set of currentlyavailable APs and the set of APs that were available at the last markedtime slot. Where this set includes more than one AP, the selection fromamong this intersection set may either be arbitrary, according to thestrongest signal, or according to any other means.

According to the example shown on FIG. 3, at time slot 2, the set ofcurrently available APs is {AP1, AP3, AP4, AP6, AP9} and the set of APsthat were available at the last marked time slot is {AP1, AP2, AP3, AP4,AP5}. Thus, the set of intersection is {AP1, AP3, AP4}. In this example,AP1 is selected from among the intersection set.

An association may last until the associated AP is no longer available.In the example at time slot 3, the currently associated AP is stillavailable and thus association is maintained. However, at time slot 4,AP1 is no longer available and a new AP is selected for handoff. Thistime, the set of available APs is {AP6, AP7, AP8, AP9} and the set ofAPs available at the last marked time slot is {AP1, AP2, AP3, AP4, AP5},thus the intersection set is null. According to this implementation ofthe lookback approach, when the intersection set is found to be null,the current time slot is marked.

When the current time slot is marked, the intersection of the currentlyavailable APs and the APs available at the most recently marked slot areidentical and thus any available time slot may be selected. In thisexample, AP6 is so selected. Then because AP6 remains available at timeslot 5, no handoff occurs.

At time slot 6, the associated AP is no longer available and thus a newAP is selected for handoff. The set of currently available APs is {AP7,AP10, AP11, AP12} and the set of APs available at the most recentlymarked time slot is {AP6, AP7, AP8, AP9}, thus the intersection set is{AP7}. Accordingly, AP7 is selected to receive handoff. Throughout timeslots 7, 8, and 9, AP7 remains available and thus no further handoffsoccur.

FIG. 4 is a flowchart illustrating a method for performing AP handoffusing a Lookback approach according to an exemplary embodiment of thepresent invention. When no prior association exists, an initialassociation may be made based on the presently available APs (Step S40).Selecting between the available APs may be arbitrary, may be based onstrongest signal, or based on another mode of selection. Also in thisstep, the present time slot may be marked.

Association may continue for as long as the mobile terminal remainswithin the coverage area of the associated AP (Yes, Step S41). When theassociated AP is no longer available (No, Step S41) then the set ofcurrently available APs is determined (Step S42). Then it may bedetermined which of the set of available APs were also available at themost recently marked time slot (Step S43). This is the intersection ofthe presently available APs and the APs that were available at the mostrecently marked time slot.

If this intersection is a null set (Yes, Step S44) and there are nopresently available APs that were also available at the most recentlymarked time slot, then the present time slot is marked (Step S45). Thenthe step of determining the intersection is performed again (Step S43)for the newly marked time slot.

If the intersection is not a null set (No, Step S44), then an AP may beselected from among the intersection (Step S46). Where there aremultiple APs in this set, the selection between the APs may bearbitrary, based on signal strength, or by some other approach.

Then, handoff may be performed to the selected AP (Step S47). Theassociation with the selected AP may continue for as long as theselected AP is available (Yes, Step S48). When the selected AP is nolonger available (No, Step S48), then the process may return to stepS42. As described above, the associated AP may be considered availablefor as long as its signal strength remains above a predeterminedthreshold level.

The Lookback approach described above is offered as an exampleapplication of the Lookback approach. This approach may be modified inmany ways. For example, in the approach discussed above, the marking isset when the intersection between currently available APs and APs thatwere available at the most recently marked time slot is null. Thisaspect may be changed and another approach to marking time slots may beused. For example, every time slot may be a marked time slot and thusthe selected AP may be selected from among the intersection of thecurrently available APs and the APs that were available at theimmediately previous time slot.

While there may be exemplary situations where the Lookback approachprovides comparable or perhaps even more frequent handoffs than theconventional approach, it is believed that on average, the Lookbackapproach provides fewer handoffs than the conventional approach in lightof the real-world trajectories followed by pedestrians and vehicles thatmay be carrying mobile devices.

Track Approach

Another online approach to AP selection is the Track approach where thetrajectory of the mobile terminal is unknown. Wireless APs periodicallysend out beacons that are used to make their presence known. Thesebeacons may allow mobile terminals to establish a connection with theAPs. By listening for beacons, the mobile terminal may determine whatAPs are available at any given moment.

Even though the trajectory of the mobile terminal is unknown, exemplaryembodiments of the present invention may be trained to recognizepatterns of AP availability to estimate which of the available APs islikely to have the longest period of association with the mobileterminal as it roams and thus, by associating with the AP likely to havethe longest period of association, the frequency of handoffs may bereduced.

One method for estimating a period of association is to use history datapreviously recorded by the mobile terminal as it roams. The history datamay be collected by the mobile terminal by analyzing the set ofavailable APs at set intervals and using this set of available APs togenerate a location profile. For example, as seen in FIG. 5, at locationL51, only access point AP56 is available. Thus the location profile isAP56. As the mobile terminal roams from location L51 to location L52along the unknown trajectory T, AP56, AP57 and AP 58 are all available.Thus the location profile becomes AP56, AP57 and AP58. The transitionfrom location profile “AP56” to location profile “AP56, AP57 and AP58”is a transition state that may be recorded. Then, as the mobile terminalcontinues along its trajectory, it is observed that at location L53,AP56 is no longer available while AP57 and AP58 continue to beavailable. Thus it is recorded that at the transition state fromlocation profile “AP56” to location profile “AP56, AP57 and AP58,” AP56has an association duration of ½ interval. The association duration issaid to be ½ because it is known that association with AP56 is availableat L51 and unavailable at L52, because it may not be known at preciselywhat point the AP becomes unavailable, it is sufficient to estimate theassociation duration to be ½.

Then, as the mobile terminal roams from location L53 to location L54, itis observed that at location L54, neither AP56 nor AP57 are availablebut AP58 remains available. Thus it is recorded that at the transitionstate from location profile “AP56” to location profile “AP56, AP57 andAP58,” AP57 has an association duration of 1.5 intervals.

Then, as the mobile terminal roams from location L54 to location L55, itis observed that at location L55, none of AP65, AP57 or AP58 isavailable. Thus it is recorded that at the transition state fromlocation profile “AP56” to location profile “AP56, AP57 and AP58,” AP58has an association duration of 2.5 intervals.

The three measures of association duration are then recorded along withthe details of the transition state. This information may then be usedat a later time when the mobile terminal again encounters the transitionstate from location profile “AP56” to location profile “AP56, AP57 andAP58.” Because this transition state is now a known state, the mobileterminal at location L52 may analyze the history information and usethis information to select from the available access points. Byreferring to the history information for the known state, the availableAP with the longest association duration may be selected and handoff maybe performed with the selected AP. For the example depicted in FIG. 5,AP58 would be selected for handoff because at 2.5, it has the longestassociation duration

FIG. 6 is a flow chart illustrating a method for performing AP handoffusing a Track approach according to an exemplary embodiment of thepresent invention. First, the mobile terminal may determine a list ofavailable APs at a first location (Step S61). Then, after roaming to asecond location, the mobile terminal may determine a list of availableAPs at the second location (Step S62). The transition from the first setof available APs to the second set of available APs may have previouslybeen encountered by the mobile terminal, and at that previous encounter,history data pertaining to the association durations of each AP may havebeen collected. Accordingly, the relevant history data pertaining to thetransition between the available APs at the first location and theavailable APs at the second location may be retrieved (Step S63). Thishistory data may include association duration information for eachavailable AP. The available AP with the greatest association durationmay then be selected (Step S64). Where the AP is selected for thepurposes of handing off network access, the network access may be handedoff to the selected AP (Step S65). After network access is establishedand/or handed off, the signal strength may be monitored to determine ifthe signal strength is within a predetermined threshold (Step S66). Foras long as the signal strength is within the predetermined threshold(Yes, S66), no handoff is necessary and monitoring may continue (StepS66). When the signal strength falls below the predetermined threshold(No, S66), then the handoff process may be repeated at Step S61.

Combination Approaches

Exemplary embodiments of the present invention may combine one or moreof the above-described approaches. For example, all three of the aboveapproaches may be selectively utilized depending on the availableinformation. For example, the Lookahead approach may be used where thetrajectory of the mobile terminal is known. If the trajectory of themobile terminal is not known and appropriate history information isavailable, then the Track approach may be used. However, if neither thetrajectory of the mobile terminal is known nor is there appropriatehistory information available, and at least the set of available APs fora prior time and/or location is known then the Lookback approach may beused. Where there is not enough information to employ the Lookbackapproach, for example, then the conventional strongest-signal approachmay be used.

FIG. 7 shows an example of a computer system which may implement amethod and system of the present disclosure. The system and method ofthe present disclosure may be implemented in the form of a softwareapplication running on a computer system, for example, a mobile computersystem. The software application may be stored on a recording medialocally accessible by the computer system and accessible via a hardwired or wireless connection to a network, for example, a local areanetwork, or the Internet.

The mobile computer system referred to generally as system 1000 mayinclude, for example, a central processing unit (CPU) 1002, randomaccess memory (RAM) 1003, a non-volatile storage unit 1004, for example,a hard drive or a flash memory, a radio transmitter, for exampleconforming to 802.11 standards, for interfacing with a wireless accesspoint, a GPS receiver 1006 for receiving signals from a satelliteconstellation for determining a present location, one or more inputdevices 1007, for example, a keyboard, pointing device, camera, etc.,and a display unit 1008. The mobile computer system may further includean internal bus 1001 for transmitting data between the variouscomponents.

The above specific exemplary embodiments are illustrative, and manyvariations can be introduced on these embodiments without departing fromthe spirit of the disclosure or from the scope of the appended claims.For example, elements and/or features of different exemplary embodimentsmay be combined with each other and/or substituted for each other withinthe scope of this disclosure and appended claims.

What is claimed is:
 1. A method for association of a mobile terminalwith an access point (AP), comprising: determining a set of availableAPs; calculating an extent of prior uninterrupted reception lookingbackwards from a point in time when the set is determined for each ofthe available APs over a recent history; and selecting an AP from amongthe set of available APs that has a greatest extent of prioruninterrupted reception, looking backwards from the point in time whenthe set is determined, and is different from a currently associated APassociating the mobile terminal with the selected AP, to the extent thatan AP that is different from the currently associated AP is among theset of available APs, wherein the extent of uninterrupted reception,looking backwards from the point in time when the set is determined, iscalculated for each of the available APs by determining how long themobile terminal has remained within the coverage area of each of the APsgiven a known or estimated trajectory of the mobile terminal.
 2. Themethod of claim 1, wherein the set of available APs includes APs of awireless network that have a coverage area that encompasses the mobileterminal at the point in time when the set is determined.
 3. The methodof claim 1, wherein selecting an AP from among the set of available APsthat has a greatest extent of uninterrupted reception, looking backwardsfrom the point in time when the set is determined, includes randomlyselecting an AP from among APs that have an identical extent ofuninterrupted reception, looking backwards from the point in time whenthe set is determined.
 4. The method of claim 1, wherein selecting an APfrom among the set of available APs that has a greatest extent ofuninterrupted reception, looking backwards from the point in time whenthe set is determined, includes selecting an AP having a greatest signalstrength from among APs that have an identical extent of uninterruptedreception, looking backwards from the point in time when the set isdetermined.
 5. The method of claim 1, wherein the association of themobile terminal with the selected AP is a handoff from the currentlyassociated AP.